Telecommunication wires today are often comprised of a copper wire surrounded by a layer of pulp insulation. During the insulation phase of its manufacture the copper wire is routed through a pulpous slurry bath and onto a cylinder mold where it is centered in a flat ribbon of wet pulp. The pulp ribbon and wire are then emersed from the bath, the ribbon spun about the wire, the now tubular shaped insulation partially dried in a furnace, and the insulated wire wound on a reel. All of this is typically done in a continuous, single pass operation at speeds of some 120 to 200 feet per minute simultaneously on a large number of wires.
More specifically, 60 copper wires are typically fed from 60 supply reels firstly through an electrolytic cleaner and then about a cylinder mold rotating in a vat of a pulpous slurry. The rotating cylinder mold is divided into 60 narrow, circumferential sections of exposed wire mesh separated by annular strips of a painted-on plastic where pulp is prevented from depositing. Individual ribbons of pulp are thus formed on the unpainted, exposed areas of the wire mesh. The copper wires are guided so that each wire is embedded in the center of one of the ribbons as they are formed. Once formed the wet, pulpous ribbons with the embedded wires are transferred from the cylinder mold onto an endless, transfer belt made of felt. The belt, ribbons and wires are then passed through a pair of rubber press rollers that press out some water from the ribbons. At this point the ribbons are about 5/16 inch wide and form an insulation thickness that varies from 0.007 to 0.016 inch, depending on wire size. The wet ribbons are next spun about the wires by passing them through a high speed, rotating polisher. Finally, the insulation is dried to about 7% moisture content by weight by passing it through a furnace. For a more detailed explanation of this process reference may be made to the July-October 1971 issue of The Western Electric Engineer and to the article appearing therein on pages 86-94 titled Manufacturing Pulp Cable by Chester Britz and William P. Klein.
During the just described process it sometimes happens that one of the wires breaks as it is being insulated. If such a break occurs at a point where the wire is adjacent to, or actually in contact with the cylinder mold, the wire will start wrapping around the mold. After a few convolutions of wire have built up on the cylinder mold the accumulation will scrape on the transfer belt, and the couch roll that guides the belt into contact with the cylinder mold, and sometimes even on the vat walls themselves. This action generates substantial noise. If the accumulation has not already been spotted by an attendant the attendant may hear this noise, realize that a wire break has occurred, and stop the manufacturing line. Unfortunately, the accumulation of broken wire will often at this point already have damaged the cylinder mold by bending and twisting it. Moreover, once several convolutions of wire have been built up the wire itself becomes quite entangled and difficult to remove from the mold. It thus would be desirable to have the wire insulation apparatus include means for detecting a wire break in a more rapid manner so that the pulp insulating apparatus or machine could be halted before substantial damage and wire removal inconvenience had occurred.
Wire and metal detectors, of course, are available for confirming the presence of wires. By and large, however, they are not suitable for incorporation into wire insulating machines of the type described. For example, with many detectors a wire under surveillance or test must be passed through an inductive, coil member of a tuned, electronic circuit that oscillates under certain structural conditions of the wire. U.S. Pat. No. 2,326,344 is exemplary of this type test procedure. Obviously, it would be almost impossible to have a substantial number of continuous wires embedded in a multiplicity of pulp ribbons upon a single, endless belt of a pulp insulating machine pass through an array of such coils. Furthermore, the electronic circuitry employed creates oscillations when a normally structured wire is present and ceases oscillations when abnormalities appear as where portions of the wire metal are absent. Thus, were the wires to be passed outside of the coils they would have to be held precisely in spatial relation with the coil to avoid erroneous signals from being generated.
Other metal detectors employ oscillator frequency change as a wire or metal detection technique. For example, U.S. Pat. No. 3,467,855 illustrates a metal detector having means for establishing a field of radiofrequency (RF) energy, which throughout this application means frequencies in excess of one megahertz, that includes an inductor coil adapted to be passed in proximity to an object to be detected. This device has an RF output signal altered by disturbances in the radiated field when the inductor is in proximity with a metallic object. The device also has means for bearing the RF output with that of a constant frequency FR output to produce an alternating current signal at a beat frequency which vary in response to frequency changes in the first RF signal caused by the disturbances. Unfortunately, this type of detector is expensive and complicated requiring the use of frequency meters, phaseshift indicators, means accomodating for oscillator frequency drift and the like. Furthermore, since a pulp insulating machine would require a multitude of such indicators in close proximity to each other, additional circuit design sohpistication would be required to avoid interference between the various fields.